The maximum diameter of a physical area for high-resolution dotting on paper is 25 μm. An electronic image detector reads such a minute image from a medium and converts the image into an electric signal which is generally in a digital data format, and then the image or data can be displayed on a screen or be replicated.
Reduction printing is mainly used in the fields of banknote printing, photocopying, watermarking and paper document security. These fields have different sensing requirements because data can be presented in various formats, for example, using magnetic ink or optical ink. Besides, the printing format is scarcely limited, and a printed character or target may appear at any position of a medium, may have any shape and may use ink or marks of any intensity.
Medium reading systems are mainly divided into two types, namely, a scan reading system and a fixed reading system. The fixed reading system can keep a reader and a medium relatively still, for example, a digital camera sensor array. The scan reading system operates by making a medium move relative to a scanning read head, for example, a flatbed scanner, a credit card reader and most of currency detectors.
Before introduction of e-readers and scanners, there is not much need to make printed matters smaller than what can be seen by the human eyes. However, with rapid development of reduction printing and scanning technologies, there is a need and an opportunity to read printed matters smaller than 25 μm, and moreover, the existing digital scanning speed is not high and the scanning quality is not good enough. As for the quality of stored scanned images, optical scanning is better than magnetic scanning. Therefore, a magnetic scanning head with higher spatial resolution is really needed, a magnetic scanning system capable of providing a magnetically printed image of high fidelity is also needed, and the fidelity of the magnetically printed image is limited to relative geometric positions of elements on the sensing system, the quantity of induction elements on the scanning head and the requirement of distributing the magnetic field of the image along multiple coordinate axes.
As for the problem in the prior art of low resolution of a magnetoresistive imaging sensor, the present invention is directed to improve the spatial resolution of an image from a magnetoresistive imaging sensor array by reducing the spacing between the magnetoresistive imaging sensor array and a medium. The spacing aspect ratio is a parameter for roughly estimating the spacing required to distinguish two different magnetic induction targets. Based on the objective of the present invention, the spacing aspect ratio is defined as: a ratio of a distance (A) between the magnetoresistive imaging sensor array and the medium to a distance (B) between the two magnetic induction targets. When the spacing aspect ratio is less than 1:1, the imaging is easy; and when the aspect ratio is greater than 1:1, image signals from two adjacent objects become blurred, and it is difficult to distinguish the image. The magnetoresistive imaging sensor array generally receives the same signals from many different places, and when the spacing aspect ratio is 10:1, the resolution is extremely low. Besides, the amplitude of a magnetic mark signal from the medium and the distance between the magnetoresistive imaging sensor array and the medium form a relationship of 1/A3, so when A is very large, not only the image is blurred but also the signal-to-noise ratio is low. Therefore, the spacing between the magnetoresistive imaging sensor array and the medium needs to be reduced as much as possible.